How do you create an animated mask for an image to show underlying video for v2.82a?
In the movie clip editor (not the sequencer) you can create and animate masks.

In the VSE select the strip you want to overlay, add a &quot;mask&quot; strip modifier and select the mask you want to use. Then set the strip to use &quot;alpha over&quot; blend mode in the Adjust-&gt;Compositng settings for the strip.
The top strip will be overlayed on the bottom strip using the mask as alpha channel.

There is no need to create black and white images to be used as masks. The mask tools for blender are quite sophisticated.